If you are a beginner in philosophy (even as a grad student I still largely consider myself a beginner), then I highly recommend this book
Philosophical Devices: Proofs, Probabilities, Possibilities, and Sets by David Papineau
Philosophical Devices teaches you about many of the things you need to know when reading and doing philosophy. Sets, possible worlds, de dicto and de re necessity, probability, analytic and synthetic truths, Bayes’ Theorem, and much, much more. When taking philosophy classes it is common to come across reading and writing assignments that use terminology and concepts that the students are not familiar with. The papers they read simply assume its readers know the material. For students and others interested in philosophy who come across this material, it is up to them to find out what the terms and concepts mean so that they can continue. This book can help you do just that.
First, the book is very readable. I was able to digest much of the material very easily. The author does not assume that the reader has prior knowledge of the concepts, terms, and logical symbols he discusses. He teaches you what they mean and what they are in a highly readable way.
Second, this book is a challenge. However, it is not challenging to the point of being an absolute misery to read through. It is just challenging enough to be very rewarding to get through without slowing you down too much. Admittedly how challenging it is will depend on how much exposure the reader has had to philosophy. I did not have much trouble getting through the material, but this book still strikes a good balance between being very readable and being just challenging enough to be a rewarding experience to understand.
All in all, I recommend this book to people going into philosophy as a very good and readable introduction to many of the concepts in philosophical discussion.